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Abstract

Reliable and cost-efficient reservoir surveillance techniques play an important role in the context of geological carbon dioxide storage. Electrical methods are particularly suited to image the displacement of highly conductive formation fluids by a more resistive CO2 plume. At the Ketzin site close to Berlin, Germany, a permanent installation of 45 ring-shaped stainless steel electrodes distributed in an injection well and two observation wells is in operation since the start of injection in June 2008. The depth of the installation intervals in the three wells ranges from 590 m to 735 m. Based on our practical experience after more than five years of injection (until August 2013), we draw conclusions with regard to the usefulness and versatility of such an electrode array as well as associated challenges under the aspect of technical longevity. With regard to future installations, we present an experimental design approach to estimate optimum electrode locations along the borehole trajectories with the objective to maximize the resolution within a prescribed target horizon. Our results show that a sparse but well-conceived set of electrodes with a refinement of the electrode spacing in the target region can provide a large portion of the information content offered by very dense electrode layouts. Using Ketzin as a case study for realistic CO2 plume migration, we compare the optimized layout to the existing equidistant installation at Ketzin with a focus on the offered resolution and the inversion performance. The presented results can assist practitioners in the design of reliable and cost-efficient monitoring systems for future underground storage sites.